Global warming, or the so-called greenhouse effect, is caused by the increase of the carbon dioxide concentration in the atmosphere. One of the significant causes for the greenhouse effect is the energy production where burning of fossile fuels generates most of the carbon dioxide accumulating in the atmosphere and also in plants through photosynthesis, or in the so-called natural recycling processes.
Carbon dioxide concentration of the atmosphere is very low, only slightly above 0.3%. Binding of carbon dioxide directly from the atmosphere would for instance be possible with washing techniques, which, however, are not feasible due to the low partial pressure of carbon dioxide. In practice, the only way for carbon dioxide removal from the atmosphere is to apply solutions based on photosynthesis by plants. In photosynthesis, sugars (C6H12O6) are generated by plants by means of solar radiation energy from the atmospheric carbon dioxide (CO2) and water (H2O) absorbed by the roots from the soil, and concomitantly, oxygen (O2) is released into the atmosphere. From the sugars, plants produce, among others, cellulose for growing cellular walls and starch for storing energy. Moreover, various other organic substances such as fats and vitamins are produced by plants from the sugars and nutrients absorbed with water.
Fossile fuels (such as coal, crude oil, and natural gas) mainly consist of carbon and hydrocarbons. Most of the man-made carbon dioxide believed to contribute to the greenhouse effect is generated by burning of these fuels. Technologies are being developed, enabling the separation, at the power plant, of the carbon dioxide generated by this burning to give a separate substance stream that may be ultimately disposed of in a landfill. These alternatives include the recovery during gasification, solutions based on combustion with oxygen, and chemical washing of flue gasses. All these alternatives share the common feature that for the recovery, the production units should have high capacities and long operation times to make the recovery at least theoretically feasible considering the price levels in the present carbon dioxide trading (10-50 /metric ton of carbon dioxide).
The term biomass refers to materials derived from living plants and generated by photosynthesis. Fuels derived therefrom are called biofuels. Bioenergy is energy from said biofuels. In Finland, biofuels are obtained from biomass growing in forests, wetlands and fields, as well as from municipal, agricultural and industrial organic wastes suitable for the production of energy. Bioenergy is one of the sources of renewable energy.
Use of bioenergy in the power production is considered to be neutral with respect to carbon dioxide emissions, and for this reason emissions therefrom are not part of the carbon dioxide trading. Carbon dioxide released during burning of biofuels is considered to be recycled to the atmosphere only via a different route than in conventional natural recycling during decomposition of the biomass.
Thermal treatment of biomass under mild conditions is called torrefaction (also known as the roasting process). This process is a thermochemical treatment normally carried out at atmospheric pressures and at temperatures of about 200-300° C. under oxygen-free conditions. During the process, water and additional volatile components present in the biomass are removed and biopolymers are partially decomposed to give volatile compounds. The final product consists of the remaining dry and solid material. Properties of the biomass for fuel application are significantly improved by the process.
The document EP 2017 325 A2 discloses an apparatus and process for decomposing organic plant material. The apparatus includes, among others, torrefaction and pyrolysis reactors. Torrefaction serves as the pretreatment stage of the organic plant material prior to pyrolysis. From the torrefaction, both the solid and gaseous materials are passed to pyrolysis. Heat is indirectly brought to the torrefaction reactor by heat conductors.
The document WO 2007/078199 A1 discloses a process and apparatus for the treatment of biomass in a torrefaction reactor comprising drying and torrefaction chambers, respectively. Both drying of biomass and torrefaction of the dried biomass are performed using hot gasses.
Technology of the torrefaction process and known processes are described by Aula Uslu in: Pre-Treatment Technologies and Their Effects on the International Bioenergy Supply Chain Logistics, Techno-economic evaluation of torrefaction, fast pyrolysis and pelletisation, Report Number: NWS-I-2005-27, December 2005.
Several torrefaction processes are already known that are, however, difficult to control and wherein the amount of biomass energy lost in the gaseous form is high.